Mobile wireless ad-hoc networks find applications in a broad range of situations, including, rescue operations, law enforcement operations, military deployment and sensor deployment. Wireless ad-hoc networks, in essence, are mobile nodes that communicate with each other. The mobility of the nodes makes the topology of the network time-variant. The rate of change of the network topology depends on a variety of factors including the velocity and relative direction of the nodes. Furthermore, wireless ad-hoc networks are generally characterized by low bandwidth links that are subject to harsh conditions of fading and interference; consequently routing in such networks is highly complex. A plethora of routing protocols have been proposed for wireless ad-hoc networks. These protocols may generally be classified as either proactive or reactive. When proactive routing protocols are employed, a node possesses routing information to a destination before it actually needs to route data to that destination. For this purpose routing tables are maintained. Route updates are exchanged periodically to reflect the changes in topological information. Popular proactive routing protocols for ad-hoc networks include the Destination Sequenced Distance Vector (DSDV) Protocol, the Wireless Routing Protocol, and the Source Tree Adaptive Routing (STAR) Protocol. Conversely, if reactive routing is used, a node would attempt to compute a route to a given destination when it needs to route data to that destination, i.e., on-demand. Numerous on-demand routing protocols have been proposed. Some of the on-demand routing protocols include the Adaptive On-Demand Distance Vector (AODV) protocol, the Dynamic Source Routing (DSR) Protocol and the Temporally Ordered Routing Algorithm (TORA).
The proactive routing protocols usually require the maintenance of routing tables and thus, in the dynamically changing mobile ad-hoc network, nodes need to exchange routing updates periodically. This exchange of route updates consumes bandwidth, and if the network is large, these control messages often contribute to a significant amount of overhead. On the other hand if on-demand routing protocols are used, when data is to be routed to a destination, a source node might be required to initiate a search for the destination. If the network is large, significant latency may be incurred before the destination is found. Thus, the scalability of both the table-driven and the on-demand routing protocols is limited. The Zone Routing Protocol (ZRP) provides a hybrid proactive/reactive routing framework in an attempt to achieve scalability. Each node would maintain routing tables that would only offer routes to a destination if the destination were to be within a certain maximum hop-count (which is called the zone radius) from the source node. If the destination were to be outside the zone radius, the source node would invoke an on-demand search mechanism called bordercasting. Bordercasting provides an efficient means for searching for a destination by sequentially using the routing tables of the intermediate relay nodes.
Existing routing protocols assume that the nodal links in the network are bi-directional in nature. However, a wireless ad-hoc network could potentially consist of a heterogeneous aggregation of nodes with differing transmittal range and reception capabilities. For instance, the transmission range of one node might be different from that of another. Thus, a node (say node A) having a transmission range that is larger than that of another node (say node B) will be able to transmit information to node B, but will be unable to receive the transmissions of node B. This results in the creation of a unidirectional link in the network.
Therefore it is desirable to have extensions to the zone routing protocol in order to provide a robust scalable framework for routing data in wireless ad-hoc networks when unidirectional links are present.